Jeremy Corbyn, the leader of Britain's opposition Labour Party, arrives at the BBC to attend various radio and television programs, in central London, May 30, 2017.Peter Nicholls

LONDON (Reuters) - Labour's Jeremy Corbyn, the man running to oust Theresa May as British prime minister, was unable to say how much his plans to reform childcare would cost as he launched the policy on Tuesday, attracting strong criticism from rivals.

Corbyn, leader of the leftist Labour Party who is standing on a platform of higher taxes on the wealthy, higher public spending and higher investment, stumbled over how much money it would take to broaden provision of free child care in a BBC radio interview.

"It will cost, erm, it will obviously cost a lot to do," he said during a prolonged awkward exchange. "I'll give you the figure in a moment."

Opinion polls show Labour have closed the gap on the Conservatives substantially during the campaign, but Corbyn's personal ratings remain low and the party has struggled to overcome longstanding doubts about its fiscal competence.

Footage of the interview then showed Corbyn thumbing the pages of his own manifesto and logging in to an iPad as he was repeatedly asked to provide a cost.

"I think what is important for the voters to understand is that if we don't invest in our children, and we don't invest in them for the future then they do less well in primary school, less well in secondary school, and less well in the future," he said.

He said all of Labour's election polices were fully costed.

The rival Conservative Party, which is trying to frame the election debate as a battle of personal competence between May and Corbyn ahead of the start of formal Brexit negotiations next month, said it showed he was a risky choice as leader.

Responding to criticism at a later campaign speech, Corbyn said: "I didn't have the exact figure in front of me so I wasn't able to answer that question, for which obviously, I apologize."

May has also faced criticism over the funding of her party's manifesto, after the Conservative manifesto was published without a detailed financial breakdown.